A flexible shaft of the type to which this invention relates is typically connected between a relatively stationary driving device such as an electric motor or a gasoline engine, and a portable driven device. A typical driven device is a rod-like hand-held concrete vibrator which is moved laterally through a body of freshly poured concrete, such as that of a footing or a wall, to agitate the concrete and eliminate voids in it. The vibrator comprises a relatively slender cylindrical casing, typically about 18 inches (455 mm) long, in which there is journaled an eccentrically weighted shaft that rotates at high speed (e.g., 10,000 rpm or higher) to impart lateral vibration to the casing. Such a vibrator must be powered from a driving device which produces on the order of a full horsepower and which would therefore be too heavy to be mounted on the hand-held vibrator itself. A flexible shaft allows the vibrator to be connected with a driving device that remains relatively stationary and at the same time allows the vibrator to be easily manipulated.
A flexible shaft comprises an elongated flexible sheath which is connected to relatively stationary parts on the driving and driven devices to be confined against rotation, and an elongated flexible core which rotates inside the sheath and which provides a rotation transmitting connection between a driving shaft of the driving device and a driven shaft of the driven device. The flexible core ordinarily comprises a closely wound helix of spring wire. Since the core must be lubricated all along its length to minimize its friction with the sheath, it should be readily removable endwise from the sheath for lubrication at regular intervals.
Every driving device suitable for a concrete vibrator has a driving shaft with an unthreaded external surface and in which there is an endwise opening well of square cross-section that provides for connection with the core of a flexible shaft. Fixed on the housing of the driving device and projecting outwardly from it is a collar which is coaxial to the driving shaft and has an inside diameter substantially larger than the diameter of that shaft. This collar provides for a nonrotatable connection between the flexible shaft sheath and the driving device and tends to protect the driving shaft when the flexible shaft is not connected to it.
The flexible shaft heretofore available for use with concrete vibrators had several disadvantages. Secured to a front end portion of its core was a ferrule-like fitting having a rear portion that was clinched to the core and a front portion which had an internal thread and which therefore could not be directly coupled to the driving shaft of the driving device. At a front end of its sheath this prior flexible shaft had a rigid sleeve or ferrule with an internally threaded front end portion, and this securement means on the sheath was likewise not suitable for direct connection to the fixed collar on the driving device housing inasmuch as that collar had unthreaded radially inner and outer surfaces. Thus, connection of the prior flexible shaft to the driving device required a separate coupling adapter which was first installed on the flexible shaft and was then connected to the driving device.
The coupling adapter comprised a sturdy tubular sleeve consisting of coaxial front and rear telescoped sleeve parts, a ball bearing in the sleeve, intermediate its ends and axially confined between internal shoulders on the respective sleeve parts, and a spindle coaxially rotatable in the ball bearing and projecting forwardly and rearwardly beyond it. Spring clips, received in circumferential grooves in the spindle at axially opposite sides of the ball bearing, confined the spindle against axial movement relative to the adapter sleeve. The front end portion of the adapter spindle had a square cross-section, to be receivable in the mating well in the shaft of the driving device;, its rear end portion had an external thread that mated with the internal thread on the ferrule-like front core fitting. An external thread on the rear end portion of the adapter sleeve mated with the internal thread on the front securement means on the sheath, and the front end portion of the adapter sleeve was formed for connection to the fixed collar on the driving device. The core of the flexible shaft was first screwed onto the adapter spindle and then the securement means on the sheath was screwed onto the rear part of the adapter sleeve.
Ordinarily the core of the flexible shaft moved easily into and out of the sheath, and allowing the core to project from the sheath risked the possibility that the lubricated core would pick up abrasive dirt. With the coupling adapter installed, the core was substantially confined against lengthwise motion relative to the sheath, and therefore the adapter was usually kept attached to the flexible shaft.
The fixed collar on the driving device has an endwise slidable detent pin extending radially through it and biased radially inwardly to a normal position in which its inner end is near the driving shaft but spaced from it. The front end portion of the adapter sleeve had a plain cylindrical surface which was axially slidably receivable in the collar and had circumferentially spaced radially outwardly opening holes in which the detent pin was receivable. Thus, with the detent pin drawn out, the adapter sleeve was slid axially into the collar at the same time that the adapter spindle was axially inserted into the well in the driving shaft, and then the detent pin was released and the adapter sleeve was rotated until that pin dropped into one of the holes in it.
Although the coupling adapter was not unduly expensive, its elimination was nevertheless obviously desirable from a cost standpoint and to avoid the inconvenience of having to secure the flexible cable to the adapter before connecting it with the driving device.
Another and perhaps more important disadvantage involved in the use of the coupling adapter was that its bearing often became highly heated when the apparatus was in use, and heat from that bearing tended to be conducted along the sheath, often causing it to become too hot to handle comfortably, even at its end connected with the driven device. Such heating was most likely to occur when the core of the flexible shaft had been elongated by use and tended to impose substantial axial thrust force upon the bearing of the coupling adapter, owing to reaction of the core against the driven shaft of the driven device and the connection of the sheath to that device.
Attached to the sheath at its rear end, that is, its end connectable to the driven device, is a sturdy tubular sleeve which is received in a fixed collar on the driven device and which has an external thread that mates with an internal thread in that collar. For connection of the flexible shaft core with the driven shaft, the driven shaft has a square cross-section, and a ferrule-like fitting, clinched to the rear end of the core, projects coaxially a substantial distance rearwardly beyond the sleeve and defines a rearwardly opening well of square cross section in which the driven shaft is axially slidably engageable and disengageable.
Although the mating square-section socket portions of the rear core fitting and of the driven shaft are of substantial axial length, to provide a driving connection between them that accommodates a substantial amount of elongation of the core relative to the sheath, the core, when elongated beyond a certain point, tended to be compressed lengthwise between the driven device and the coupling adapter, and the compressive force upon it was imposed upon the bearing of the coupling adapter and tended to heat it.